In studies of sera from 11 patients with acquired immunodeficiency disease (AIDS), transforming Epstein-Barr virus (EBV) was isolated from the plasma of four. The latter expressed HTLV-III antibody as well as significantly elevated levels of EBV antibodies. Uncultured peripheral mononuclear cells from these individuals contained 8-15% EBV-membrane antigen (MA)-positive cells. Transforming virus was isolated at various passage levels after culture. In comparison, healthy donors not at risk of developing AIDS did not contain MA-positive cells in the peripheral blood and no EBV could be isolated from plasma or cultured cells. None of the EBV-positive AIDS sera contained neutralizing antibody to EBV which were present in sera from healthy donors. This is the first report of the isolation of EBV from the plasma of AIDS patients. In response to a request from NASA, squirrel monkeys which were to be used on space lab flights 3 and 4 were tested for Herpesvirus saimiri (HVS) which is endemic in this species. NASA officials were concerned about the possibility of crew members being infected by HVS being shed by he monkeys within the close confines of the spacecraft. Sera of 150 individuals occupationally exposed to squirrel monkeys were tested for the presence of HVS, and 11 (7.3%) were found positive for antibodies to HVS late antigen (LA) in the immunofluorescence test. No other HVS antibodies were evident. Eight of the 11 sera also immunoprecipitated HVS proteins. These results suggest that replication of HVS either does not occur in humans, or is too minimal to elicit immune responses. Although no demographic patterns in the HVS-LA positives were noted, the majority (7/11) had worked at primate centers around the country for periods ranging from less than one year to 25 years. This was the first report of the detection of HVS antibodies in humans.